<!doctype html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport"
          content="width=device-width, user-scalable=no, initial-scale=1.0, maximum-scale=1.0, minimum-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
    <style>
        button {
            --border-radius: 15px;
            --border-width: 4px;
            appearance: none;
            padding: 1em 2em;
            position: relative;
            border: 0;
            background-color: #212121;
            font-family: "Roboto", Arial, "Segoe UI", sans-serif;
            font-size: 18px;
            font-weight: 500;
            color: #fff;
            z-index: 2;
        }

        button::after {
            --m-i: linear-gradient(#000, #000);
            --m-o: content-box, padding-box;
            content: "";
            position: absolute;
            left: 0;
            top: 0;
            width: 100%;
            height: 100%;
            padding: var(--border-width);
            border-radius: var(--border-radius);
            background-image: conic-gradient(
                    #488cfb,
                    #29dbbc,
                    #ddf505,
                    #ff9f0e,
                    #e440bb,
                    #655adc,
                    #488cfb
            );
            -webkit-mask-image: var(--m-i), var(--m-i);
            mask-image: var(--m-i), var(--m-i);
            -webkit-mask-origin: var(--m-o);
            mask-origin: var(--m-o);
            -webkit-mask-clip: var(--m-o);
            mask-composite: exclude;
            -webkit-mask-composite: destination-out;
            filter: hue-rotate(0);
            animation: rotate-hue linear 500ms infinite;
            animation-play-state: paused;
        }
        button:hover::after {
            animation-play-state: running;
        }

        @keyframes rotate-hue {
            to {
                filter: hue-rotate(1turn);
            }
        }

        button,
        button::after {
            box-sizing: border-box;
        }

        button:active {
            --border-width: 5px;
        }
    </style>
    <link rel="stylesheet" href="/common.css">
</head>
<body>
<button>Hover me!</button>
</body>
</html>